Referring to FIG. 1, a conventional down-conversion mixer includes a single-ended to quadrature differential converting circuit 11, a single-ended to differential converting circuit 12, a transconductance circuit 13, a mixing circuit 14 and a buffering circuit 15.
The single-ended to quadrature differential converting circuit 11 converts a single-ended oscillatory voltage signal (Sos) into a first differential oscillatory voltage signal pair and a second differential oscillatory voltage signal pair. The single-ended to differential converting circuit 12 converts a single-ended input voltage signal (Sr) of radio frequency into a differential input voltage signal pair. The transconductance circuit 13 includes four transistors 131-134 and an inductor 135, and converts the differential input voltage signal pair into a differential input current signal pair (I1, I2). The mixing circuit 14 includes eight transistors 141-148 and four resistors 149, and mixes the differential input current signal pair (I1, I2) with the first and second differential oscillatory voltage signal pairs to generate a first differential mixed voltage signal pair and a second differential mixed voltage signal pair that are of intermediate frequency. The buffering circuit 15 buffers the first and second differential mixed voltage signal pairs to generate a first differential output voltage signal pair (Vo11, Vo12) and a second differential output voltage signal pair (Vo21, Vo22).
When a frequency of each of the first and second differential mixed voltage signal pairs is low, a conversion gain (CG) of a combination of the transconductance circuit 13 and the mixing circuit 14 can be expressed by the following equation:
                                          C            ⁢                                                  ⁢            G                    ≈                                                    2                Π                            ·              sin                        ⁢                                                  ⁢                                          c                ⁡                                  (                                      Π                    ·                                          Δ                      /                                              T                        OS                                                                              )                                            ·                              (                                                      g                                                                  m                        ⁢                                                                                                  ⁢                        131                                            ,                      132                                                        +                                      g                                                                  m                        ⁢                                                                                                  ⁢                        133                                            ,                      134                                                                      )                            ·                              R                L                                                    ,                            Equation        ⁢                                  ⁢        1            where gm131,132 denotes a transconductance of each of the transistors 131, 132, gm133,134 denotes a transconductance of each of the transistors 133, 134, RL denotes a resistance of each of the resistors 149, TOS denotes a period of each of the first and second differential oscillatory voltage signal pairs, and Δ denotes a half of the fraction of the period in which all of the transistors 141-148 conduct.
Since all of the parameters in Equation 1 are fixed, both the CG of the combination of the transconductance circuit 13 and the mixing circuit 14 and a CG of the conventional down-conversion mixer are fixed. In addition, the CG of the conventional down-conversion mixer is low, and a noise figure of the conventional down-conversion mixer is high.